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"""
.. _ex-time-freq-global-field-power:
===========================================================
Explore event-related dynamics for specific frequency bands
===========================================================
The objective is to show you how to explore spectrally localized
effects. For this purpose we adapt the method described in [1]_ and use it on
the somato dataset. The idea is to track the band-limited temporal evolution
of spatial patterns by using the :term:`Global Field Power(GFP) <GFP>`.
We first bandpass filter the signals and then apply a Hilbert transform. To
reveal oscillatory activity the evoked response is then subtracted from every
single trial. Finally, we rectify the signals prior to averaging across trials
by taking the magniude of the Hilbert.
Then the :term:`GFP` is computed as described in [2]_, using the sum of the
squares but without normalization by the rank.
Baselining is subsequently applied to make the :term:`GFPs <GFP>` comparable
between frequencies.
The procedure is then repeated for each frequency band of interest and
all :term:`GFPs <GFP>` are visualized. To estimate uncertainty, non-parametric
confidence intervals are computed as described in [3]_ across channels.
The advantage of this method over summarizing the Space x Time x Frequency
output of a Morlet Wavelet in frequency bands is relative speed and, more
importantly, the clear-cut comparability of the spectral decomposition (the
same type of filter is used across all bands).
We will use this dataset: :ref:`somato-dataset`
References
----------
.. [1] Hari R. and Salmelin R. Human cortical oscillations: a neuromagnetic
view through the skull (1997). Trends in Neuroscience 20 (1),
pp. 44-49.
.. [2] Engemann D. and Gramfort A. (2015) Automated model selection in
covariance estimation and spatial whitening of MEG and EEG signals,
vol. 108, 328-342, NeuroImage.
.. [3] Efron B. and Hastie T. Computer Age Statistical Inference (2016).
Cambrdige University Press, Chapter 11.2.
""" # noqa: E501
# Authors: Denis A. Engemann <denis.engemann@gmail.com>
# Stefan Appelhoff <stefan.appelhoff@mailbox.org>
#
# License: BSD (3-clause)
import os.path as op
import numpy as np
import matplotlib.pyplot as plt
import mne
from mne.datasets import somato
from mne.baseline import rescale
from mne.stats import bootstrap_confidence_interval
###############################################################################
# Set parameters
data_path = somato.data_path()
subject = '01'
task = 'somato'
raw_fname = op.join(data_path, 'sub-{}'.format(subject), 'meg',
'sub-{}_task-{}_meg.fif'.format(subject, task))
# let's explore some frequency bands
iter_freqs = [
('Theta', 4, 7),
('Alpha', 8, 12),
('Beta', 13, 25),
('Gamma', 30, 45)
]
###############################################################################
# We create average power time courses for each frequency band
# set epoching parameters
event_id, tmin, tmax = 1, -1., 3.
baseline = None
# get the header to extract events
raw = mne.io.read_raw_fif(raw_fname)
events = mne.find_events(raw, stim_channel='STI 014')
frequency_map = list()
for band, fmin, fmax in iter_freqs:
# (re)load the data to save memory
raw = mne.io.read_raw_fif(raw_fname, preload=True)
raw.pick_types(meg='grad', eog=True) # we just look at gradiometers
# bandpass filter
raw.filter(fmin, fmax, n_jobs=1, # use more jobs to speed up.
l_trans_bandwidth=1, # make sure filter params are the same
h_trans_bandwidth=1) # in each band and skip "auto" option.
# epoch
epochs = mne.Epochs(raw, events, event_id, tmin, tmax, baseline=baseline,
reject=dict(grad=4000e-13, eog=350e-6),
preload=True)
# remove evoked response
epochs.subtract_evoked()
# get analytic signal (envelope)
epochs.apply_hilbert(envelope=True)
frequency_map.append(((band, fmin, fmax), epochs.average()))
del epochs
del raw
###############################################################################
# Now we can compute the Global Field Power
# We can track the emergence of spatial patterns compared to baseline
# for each frequency band, with a bootstrapped confidence interval.
#
# We see dominant responses in the Alpha and Beta bands.
# Helper function for plotting spread
def stat_fun(x):
"""Return sum of squares."""
return np.sum(x ** 2, axis=0)
# Plot
fig, axes = plt.subplots(4, 1, figsize=(10, 7), sharex=True, sharey=True)
colors = plt.get_cmap('winter_r')(np.linspace(0, 1, 4))
for ((freq_name, fmin, fmax), average), color, ax in zip(
frequency_map, colors, axes.ravel()[::-1]):
times = average.times * 1e3
gfp = np.sum(average.data ** 2, axis=0)
gfp = mne.baseline.rescale(gfp, times, baseline=(None, 0))
ax.plot(times, gfp, label=freq_name, color=color, linewidth=2.5)
ax.axhline(0, linestyle='--', color='grey', linewidth=2)
ci_low, ci_up = bootstrap_confidence_interval(average.data, random_state=0,
stat_fun=stat_fun)
ci_low = rescale(ci_low, average.times, baseline=(None, 0))
ci_up = rescale(ci_up, average.times, baseline=(None, 0))
ax.fill_between(times, gfp + ci_up, gfp - ci_low, color=color, alpha=0.3)
ax.grid(True)
ax.set_ylabel('GFP')
ax.annotate('%s (%d-%dHz)' % (freq_name, fmin, fmax),
xy=(0.95, 0.8),
horizontalalignment='right',
xycoords='axes fraction')
ax.set_xlim(-1000, 3000)
axes.ravel()[-1].set_xlabel('Time [ms]')
|
